If you are operating in a mostly Mac environment, ProRes is still fantastic. It can get a bit tricky, though, if you need to export ProRes on a PC. Yes, there are third-party options, but they were essentially created by reverse-engineering Apple's codecs, and I've seen scenarios where some really bizarre problems came up.
If you are working with both PC and Mac and aren't working with resolutions above 1080p, DNxHD is fantastic. One can think of it as Avid's equivalent to ProRes, and it's great... but until DNxHR is fully out and adopted, no 4K for you.
If you're working with 2K/4K in a PC/Mac environment, Cineform does seem to be a great intermediate option. It's a newer option, but I know a lot of folks that adopted it for this very purpose, apparently with good results, myself included.
I've never been convinced that a download equates to a lost sale. People who download your film would probably have just not seen it otherwise. But if you read the comments on TPB, you'll notice that plenty of these folks genuinely love the films. Maybe they can't afford to buy it, maybe it isn't legally available in a format they want it in. Maybe they just don't want to. But as long as the content creator isn't going bankrupt as a result, I like to see people genuinely enjoying films, whether or not they paid money for it.
Practice! Over time, when you do a lot of shooting, these concerns become instinct and you get much, much faster at them. Shoot as often as you can and you'll see!
The issue with RED (and most cameras now, really) is that the camera may seem affordable, but you'll end up spending 2-3x for the kit/accessories/etc. 10k isn't going to get you a RED kit. Explain more of what you plan to shoot (and what kind of editing hardware you have) and we can help you out with something you can afford!
There is a "mind-boggling" improvement to get a PC -- performance. Putting a Titan in a PC (or now, the insanely good, inexpensive GTX 980) will put the M295X to shame. It is unlikely that the new iMac will be able to handle very heavy-duty 4K work, and it certainly will not do it as well as a similarly-priced PC with a killer video card. Also, yeah, you'll be limited by the lack of CUDA support, which you want. Adobe will be perform worse, even if the specs were otherwise the same, which they aren't.
Trust me, I used to be all Mac, I get it. It would be great if they made the best editing machines on the planet again, but they just don't. Period. If you want the Mac experience and you're not throwing anything too heavy at it, then sure, the iMac is nice. But if you are truly looking for the best 4K editing system for your money, it's not going to be this. And sadly, no amount of money put into upgrades to the iMac will make it sufficient.
Your field of vision is extremely wide, much wider than 2.35:1. If your film is going to be shown on a humongous movie theater screen, then shooting 2.35:1 will more closely mimic your actual vision, and you'll have the physical space to get away with this. If you're looking at it on your phone, suddenly 2:35:1 is far too small because of the overall size limitation.
Same reason people wanted 4:3 movies when their TVs were small, but wanted 16:9 as they became larger. Same reason why Instagram using square photos works alright for a 4" screen in portrait mode, but would look completely absurd in a theatrical setting.
16:9 is the gold standard right now; 2:35:1 could allow for a better viewing experience on a huge screen. Consider the detail you want to convey in your shot, and the viewing format the film is intended for.